MACS Monthly Newsletter - 2021 - SEP5

Figure 8: Control valve circuit codes.
Control Valve Circuit Electronic Diagnostics
* Type #1
Figure 9 shows the compressor control circuit for the
compressor control valve on a 2018 Ford F-150 pickup truck.
Many domestic vehicles use this circuit design to control
many diff erent solenoids such as fuel injectors, vent valves,
purge valves, and transmission solenoids.
Figure 10: 2018 F-150 control valve resistance.
ground to the solenoid. When activated, the solenoid will
make a " ticking " noise. If stuck by debris, the solenoid will
remain silent when activated.
The first test for the control valve circuit is to verify the supply
voltage from Fuse # 36. If voltage is present with the engine
running and A/C system turned ON, verify integrity of
the supply circuit by applying a load using a power probe or
Figure 9: 2018 F-150 control valve wiring diagram.
Fuse # 36 supplies power to the compressor solenoid from
the underhood battery junction box (BJB). Based on input
from the evaporator temperature sensor, in-vehicle temperature
and humidity sensor, sun load sensor, ambient temperature
sensor and others, the FCIM calculates the desired
compressor stroke. The desired pulse width for the solenoid
is then sent to the PCM. The PCM then modulates the ground
signal applied to the compressor control valve.
First step, measure the resistance of the control valve.
Many are designed with a coil resistance of between 10 and
15 ohms. This F-150 control valve measures at 13.3Ω. Disconnect
the solenoid connector if accessible or measure from
the controlling module plug. See Figure 10.
CAUTION: Applying straight power and ground to the
valve can burn out the solenoid winding. DO NOT test by applying
continuous 12 volts and ground!
Hint: The control valve can become " stuck " if a piece of debris
enters the sliding spool valve. See the April 2021 MACS
Service Report for a picture of the ports and filter incorporated
into its design. A quick test for a stuck valve is to use a
fuel injector pulse tester. These will safely apply power and
September 2021
5
Figure 11: 2018 F-150 control valve supply voltage.
sealed beam. See Figure 11.
On this 2018 F-150, the control circuit (via the PCM) applies
ground at 400Hz. The frequency of the signal doesn't
change so either a lab scope or DVOM capable of measuring
the PULSE-WIDTH modulation of the signal works well.
If you are an experienced scope user, these signals can also
be current ramp tested. However the circuit may need to be
accessed remotely, such as near the module controlling the
circuit, due to close quarters of the compressor and space
limitations.
The vehicle and system should be running during testing.
Like the performance test where we used pressure to determine
if the compressor was responding, now we test the system
electrical responses based on basically the same routine.
In Figure 12, you can see the solenoid ground signal applied
to the solenoid control circuit. In the scope pattern, the
MACS Service Reports
Mitchell Pro Demand
Mike Bailey
Mike Bailey
Mike Bailey
MACS Monthly Newsletter - 2021

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